1. Field of the Invention
The present invention relates to a mobile communication system and method for implementing soft handoff, and more particularly, to a mobile communication system and method that supports soft handoff between mobile switching stations.
2. Background of the Related Art
Generally, handoff is implemented in such a manner that a mobile station currently in service detects the strength of pilot signals from a serving base station and detected neighboring or remaining base stations. The serving base station is a base station providing service (e.g., a call) to the mobile station. The mobile station compares the strength of the pilot signals from the detected base stations with the strength of the pilot signal of the current serving base station to ensure readiness for obtaining a new traffic channel. If the strength of the pilot signal of the current serving base station becomes lower than a certain value, a target base station capable of maintaining a traffic channel currently in service is selected, and a call is switched into the selected target base station. Accordingly, the mobile station can continue to perform communication using the new traffic channel.
In a cellular/personal communication service (PCS) system, there are hard handoffs and soft handoffs. In the hard handoff, communication is temporarily disconnected during the call conversion. The hard handoff occurs between base stations to which different frequencies are assigned or between different mobile switching stations. In the soft handoff, communication is not disconnected. The soft handoff for a particular mobile station occurs between base stations to which the same frequencies are assigned.
FIG. 1 shows soft handoff between two base stations in a mobile communication system. As shown in FIG. 1, a first mobile station MS1 currently in service is moving from a first base station BS1 to a third base station BS3. A second base station BS2 is a neighboring base station. In the soft handoff, traffic channels between the base station BS1 and the mobile station MS1 and between the base station BS3 and the mobile station MS are respectively set at a boundary between the base station BS1 and the base station BS3.
If a signal provided through a traffic channel between the base station BS1 and the mobile station MS1 becomes equal to or below a prescribed threshold value as the mobile station MS1 moves from the first base station BS1 toward the base station BS3, the traffic channel between the base station BS1 and the mobile station MS1 is disconnected while the traffic channel between the base station BS3 and the mobile station MS1 is connected to implement the soft handoff. In such a soft handoff, the base station BS1 and the base station BS3 are controlled by the same base station controller (not shown). The base station BS1 and the base station BS3 are physically connected to each other through the base station controller.
FIG. 2 shows hard handoff between two base stations in a mobile communication system. With respect to FIG. 2, the hard handoff between the base stations to which different frequencies are assigned will be described.
As shown in FIG. 2, a first mobile station MS1 currently in service is moving from a third base station BS3 to a fourth base station BS4. In the hard handoff, at the boundary the mobile station MS1 drops a traffic channel with the base station BS3 and implements handoff using a new traffic channel from the base station BS4. The base station BS4 cannot detect the pilot signal of the base station BS3.
The related art hard handoff procedure between mobile switching stations will now be described with reference to FIG. 3. FIG. 3 is a block diagram of respective mobile switching stations.
Referring to FIG. 3, first and second access switching subsystems for wireless subscribers (ASS-W) 32 and 42 perform a call processing function and handoff and mobility management functions for the mobile station MS1. First and second access switching subsystems for wireless subscribers (ASS-T) 35 and 45 match first and second mobile switching stations 30 and 40 with a public switching telephone network 50 to perform call processing for a trunk, as shown in FIG. 3.
The first and second interconnection network subsystems 33 and 43 perform call processing functions such as mobile station number translation, route control function and switching functions. First and second central control subsystems 34 and 44 perform management, maintenance, charging and statistic functions of the system, process location registration of mobile subscribers and store/manage subscribers' information. The first mobile switching station 30 and the second mobile switching station communicate with a home location register 60 to determine and register the location of the mobile station MS1.
In FIG. 3, it is assumed that a mobile station is moving from a service area of a first and third base station 10 of the first mobile switching station 30 to a service area of a second and fourth base station 20, as shown in FIGS. 1 and 2, respectively, of the second mobile switching station 40. Initially, the first base station BS1 is the serving base station 10 of the mobile station MS1 for FIG. 1, or the third base station BS3 is the serving base station 10 of the mobile station MS1 for FIG. 2.
With reference to FIGS. 2 and 3, the mobile station currently in service periodically measures the strength of the pilot signal provided from the third base station 10 where a speech path is currently set and the strength of the pilot signal provided from a candidate base stations. The mobile station further compares the respective pilot signals provided from the candidate base stations with a preset threshold value. As a candidate base station, if the strength of the pilot signal provided from the fourth base station 20 is greater than the threshold value, the mobile station transmits the measurement results to its base station (i.e., base station 10) together with a pilot strength measurement message (PSSM).
The mobile station currently in service periodically measures the strength of a pilot signal of an active set, which includes active base stations where a speech path is set, and the strength of a pilot signal of a candidate set that includes candidate base stations received in the mobile station that can completely be modulated as a forward traffic channel. The mobile station further periodically measures the strength of a pilot signal that is not included in the current active set or candidate set or the strength of a pilot signal of a neighboring set, which includes neighboring base stations that can become a handoff candidate set. Then, the mobile station transmits the measurement results to its base station together with the PSMM.
The third base station 10 detects the fourth base station 20 toward which the mobile station is moving in response to the strength of the pilot signal included in the PSMM. If a corresponding mobile station departs from the third base station 10 service area, the third base station 10 transmits the PSMM to the first base station controller 31. However, if it is determined that the base station corresponding to the received PSSM is not under the control of the first base station controller 31, the first base station controller 31 detects the fourth base station 20 and transmits a handoff request signal to the second mobile switching station 40. The handoff request signal is transmitted to the second base station controller 41 and the fourth base station 20 through a first ASS-W 32, a first interconnection network subsystem 33, a first ASS-T 35, a public switching telephone network 50, a second ASS-T 45, a second interconnection network subsystem 44, and a second ASS-W 42.
The mobile station releases a speech path set between the mobile station and the third base station 10 under the control of the first base station controller 31. The fourth base station 20 sets a new traffic channel with the mobile station in response to the handoff request signal of the second base station controller 41. In the hard handoff between mobile switching stations, communication drops temporarily occur.
As described above, the related art handoff of mobile stations have various disadvantages. In the related art hard handoff method between the mobile switching stations, a communication protocol for implementing soft handoff between base station controllers of the respective mobile switching stations is not supported. Therefore, a hard handoff request signal is transmitted from the serving base station controller to the target base station controller through a public switching telephone network. As a result, the target base station controller that receives the handoff request signal assigns a traffic channel to the mobile station through one base station. Since the mobile station drops the traffic channel with the serving base station and maintains communication through a new traffic channel assigned from the target base station controller, there is a problem that communication drop occurs temporarily. This provides poor quality of service during communication through the mobile stations. In a particular bad result, hard handoff fails during communication, thereby unexpectedly disconnecting communication.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.